This invention relates generally to flowmeters and more specifically to flow dividers used within flowmeters to shunt a portion of a fluid flow from a main conduit.
In various flowmeter arrangements, it is desirable to measure the flow of a fluid within a main conduit. Certain flowmeter arrangements shunt a portion of the fluid flow away from the main conduit, measure either the shunted fluid flow or the unshunted fluid flow as a representative sample, and then combine the two fluid flows back into the main conduit. If the flowmeter is properly calibrated, the flow rate of the representative sample will provide an indication of the flow rate within the main conduit.
Mass flowmeters of the heated conduit type in particular, force a fluid flow in a main conduit to be measured through a flow divider so that a portion thereof flows through a heated conduit sensor section. The heated conduit sensor section measures a small amount of flow, usually in the 10-50 SCCM range. Since the flow characteristic is linear and the conduit very long compared to its diameter, it exhibits a linear relationship of mass flow vs pressure drop (P) required to create that flow, in its normal working range.
One example of a heated conduit-type flowmeter arrangement is shown in U.S. Pat. No. 4,041,757--Baker et al, commonly assigned with the present application. In U.S. Pat. No. 4,041,757, a portion of the fluid streaming through a main supply line is diverted in a shunt path by a conduit which is electrically heated from a constant power source. A thermoelectric device associated with the shunt conduit measures the temperatures at points along the shunt. These temperatures are related to the flow rate through the shunt and therefore provide an indication of the flow rate within the main conduit. There are other flowmeter arrangements that similarly depend upon flow dividers for measuring the flow of a representative sample of fluid shunted away from the flow through a main supply line.
Other known flow dividers are illustrated by U.S. Pat. No. 3,559,482--Baker et al (1968); and U.S. Pat. No. 3,851,526--Drexel (1973). These flow dividers utilize multiple capillary-like tubes having similar flow characteristics to one another. In order to alter the range of the flowmeter, the capillary-like matrix must be changed in steps by substituting tubes. Thus, it is not possible to smoothly change the flow dividing ratio.
Another known flow divider is illustrated by U.S. Pat. No. 3,805,610--Jacobs (1974) which utilizes a porous sintered material in which the porosity and/or effective working area is altered in order to change the flow range of the shunt. Its range is adjustable by changing the portion of the porous material extending beyond an annular ring (rib). However, if the annular ring is made of metal, then attempting to alter the range of the shunt can deform the outer surface of the porous sintered material permanently eliminating much of the porosity and rendering it useless. If the annular ring were fabricated from an elastic material, the usefulness of the shunt would be limited to those fluids that would not attack that particular elastomer from which the ring was fabricated.
The obvious approach to providing a smoothly adjustable flow divider would be to provide a porous threaded sleeve or tube associated with an adjustable threaded plug to change the working area of the divider. This is impractical since threading the porous material would deform it thereby, closing the pores, and rendering it virtually useless. The present invention is particularly directed to overcoming the problem just described so as to produce an adjustable flow divider having distinct operational advantages.